This invention relates generally to apparatuses and methods for inspecting and/or characterizing materials, and more particularly to ultrasonic imaging of such materials in combination with optical microscopy devices. The invention also relates to inspection system calibration methods.
The term xe2x80x9cacoustic microscopyxe2x80x9d traditionally referred to the use of high frequency ultrasound to probe the microstructural form and composition of an object. Traditional pulse/echo techniques were employed. Measurement of acoustic amplitude and phase were done either by contacting ultrasonic probes, or by optical interferometry through a microscope. Data for a particular point on a subject surface were obtained and useful images of the overall surface could be built up by raster scanning the object beneath the microscope.
The prevalence of micrometer scale mechanical features, microelectromechanical structures (MEMS), integrated circuits, microstructured materials, etc. is increasing. An accompanying need exists to probe the surface and subsurface physical, mechanical, and defect characteristics of such features, devices, and materials. However, conventional acoustic microscopy does not possess adequate resolution to produce reliable images and does not produce a real-time image of a surface since data is compiled over time from individual analysis points. Accordingly, the usefulness of conventional techniques are limited.
Telschow, et al., xe2x80x9cUHF Acoustic Microscopic Imaging of Resonator Motion,xe2x80x9d 2000 IEEE Ultrasonics Symposium Proceedings, October 22-25, Vol. 1, 631-634 (2000), describe a beginning attempt at overcoming the deficiencies of acoustic microscopy indicated above and otherwise recognized by those of ordinary skill. Telschow, et al. acknowledge the desirability of obtaining images of ultrasonic motion over an entire surface of a subject within a single video frame. The reference also indicates success at megaHertz (MHz) frequencies, although not using microscopic techniques. As acoustic wavelengths get smaller (due to increasing frequency), they become capable of detecting smaller features and defects. MHz frequencies are but a beginning step in developing significantly useful imaging technology.
There exists a heretofore unrealized need to provide methods and apparatuses for accomplishing full-field, real-time imaging developed on the foundational principles of acoustic microscopy. While the advances to-date have provided improvements, they have not yet enabled one of ordinary skill to take the next leap into microscopic imaging with frequencies high enough to probe at useful resolutions.
According to one aspect of the invention, an inspection system calibration method includes: producing two sideband signals of a first wavefront; interfering the two sideband signals in a photorefractive material, producing an output signal therefrom having a frequency and a magnitude; and producing a phase modulated operational signal having a frequency different from the output signal frequency, a magnitude, and a phase modulation amplitude. The method also includes determining a ratio of the operational signal magnitude to the output signal magnitude, determining a ratio of a 1st order Bessel function of the operational signal phase modulation amplitude to a 0th order Bessel function of the operational signal phase modulation amplitude, and comparing the magnitude ratio to the Bessel function ratio.
According to another aspect of the invention, an inspection system calibration method includes: phase modulating a reference wavefront; producing two sideband signals of the phase modulated reference wavefront; interfering the two sideband signals in a photorefractive material, producing an output signal therefrom having a frequency and a magnitude; and interfering the phase modulated reference wavefront with an object wavefront in a photorefractive material and producing an operational signal therefrom having a frequency different from the output signal frequency, a magnitude, and a phase modulation amplitude. The method also includes the comparison of the magnitude ratio to the Bessel function ratio described above.
According to a further aspect of the invention, an inspection system calibration method includes: phase modulating a reference beam with a phase modulator; amplitude modulating a drive signal of the phase modulator and producing two sideband signals of the phase modulated reference beam; interfering the two sideband signals in a photorefractive material, producing an output signal therefrom having a frequency and a magnitude, the magnitude being measured as a root-mean-square voltage; passing an object beam though an objective of an optical microscope comprised by the inspection system and reflecting the object beam from a vibrating surface that phase modulates the object beam; and interfering the phase modulated reference beam with the phase modulated object beam in the photorefractive material, producing an operational signal therefrom having a frequency different from the output signal frequency, a magnitude measured as a root-mean-square voltage, and a phase modulation amplitude. The method also includes determining a ratio of the operational signal voltage to the output signal voltage, determining a ratio of a 1st order Bessel function of the operational signal phase modulation amplitude to a 0th order Bessel function of the operational signal phase modulation amplitude, and comparing the voltage ratio to the Bessel function ratio.
According to a still further aspect of the invention, an inspection method includes: providing an object wavefront that illuminates a vibrating surface of a subject and directing a modulated object wavefront returned from the vibrating surface to a photorefractive material; phase modulating a reference wavefront; producing two sideband signals of the modulated reference wavefront; interfering the modulated reference wavefront with the modulated object wavefront in the photorefractive material, and producing an inspection signal of the vibrating surface having a frequency, a magnitude, and a phase modulation amplitude; and interfering the two sideband signals in the photorefractive material, producing an output signal therefrom having a frequency different from the inspection signal frequency and a magnitude. The method also includes calibrating by determining a ratio of the inspection signal magnitude to the output signal magnitude, determining a ratio of a 1st order Bessel function of the inspection signal phase modulation amplitude to a 0th order Bessel function of the inspection signal phase modulation amplitude, and comparing the magnitude ratio to the Bessel function ratio.